Male rats of the Sprague-Dawley strain are being used to investigate the changes in capacity and character of protein synthesis in liver as a function of age. For this purpose, hepatocytes are isolated, incubated and fractionated, etc. simultaneously from two animals of disparate ages that are to be compared. Experiments are designed to detect age-associated changes in the rates of synthesis and/or turnover of proteins of mitochondria and other subcellular fractions.